Recently, there are worries about global warming due to greenhouse effect gas (i.e., carbon dioxide), hereafter efforts for low carbonization and energy conservation have been important to maintain developable society structure.
Especially, under such condition, escape from dependence on fossil fuel is urgent, electric power generation by photoelectric conversion of solar light energy, i.e., a solar cell is a key point to solve the above problem.
Currently, raw material of the almost solar cell is silicon (Si). Meanwhile, in use of requiring high photoelectric conversion efficiency, for example in use of a solar cell mounted on satellites, a solar cell using gallium arsenide (GaAs) of chemical compound semiconductor has been practical to match solar light spectrum.
Next, an example of such solar cell is described with reference to FIG. 27. FIG. 27 is one example thereof and is a block diagram showing a configuration of a solar cell 100 of conventional semiconductor p-n junction type. The solar cell 100 comprises an n-type semiconductor layer 102, and a p-type semiconductor layer 104 formed on the n-type semiconductor layer 102. These are formed as the solar cell (unit cell) by conventional semiconductor production technique or the like.
However, the solar cell 100 uses only wavelength range of solar light corresponding to band gap energy which constituent material includes. Therefore, the solar cell having unit cell is not compatible with wide range of solar light spectrum, thereby limiting the photoelectric conversion efficiency.
Accordingly, solar cell structure of a tandem-type is proposed to widen wavelength range compatible with solar light spectrum, and to enhance photoelectric conversion efficiency. In the tandem-type solar cell, unit cells similar to the solar cell 100 shown in FIG. 27 are electrically and optically connected in series and each unit cell has different energy band gap. The unit cells are layered to be from wide energy band gap to narrow energy band gap by turns from light receiving surface along entering direction of light. That is, the tandem-type structure comprises a plurality of cells similar to the solar cell 100 shown in FIG. 27, each cell has different energy band gap. The tandem-type structure are layered to be from wide energy band gap to narrow energy band gap by turns from light receiving surface along entering direction of light.
For example, in patent reference 1, a tandem-type solar cell is compatible with solar light spectrum of wide wavelength range, since energy band gap is corresponding from 3.4 eV to 0.7 eV.